Otto Pohl found the average amount of salt in the Winsford brine to be 26.25-26.50 per cent. by weight, receding during rainy weather to 25. Thos. Ward states that the Cheshire brines usually contain about 25 per cent. (this is perhaps rather low for an average). As may be seen from the tables of solubility on p. 1711, the total solubility of pure sodium chloride in water at ordinary temperatures is about 27 per cent. The Cheshire brine rnay consequently be considered as a nearly but not quite saturated solution of common salt, especially when we remember that the small quantities of other salts present tend if anything to diminish the solubility of the sodium chloride. All natural brines employed in the manufacture of white or refined salt, whether derived from saline springs, from the concentration of sea-water, or by the direct solution of rock salt, are contaminated with other saline constituents, of which the principal are magnesium chloride, and calcium sulphate ; and it is a fact never to be lost sight of that all such brines, no matter what proportion of salt they may hold in solution, are as a rule saturated solutions of calcium sulphate. Were it not for tbe baneful presence of this calciurn sulphate, and the pan incrustation chiefly due to it, the cost of producing white salt by artificial evaporation might probably be reduced by nearly one-third. Besides these salts, natural brines contain varying but small quan tities of bromine, occasionally iodine, salts of potassium, and traces of iron, alurnina, silica, boracic and phosphoric acids, Sm. These other salts, however, interfere but little with the work of the salt maker. In Cheshire, the brine is merely first pumped, into reservoirs placed at such levels as to be able to feed the pans by an outflow-pipe. In these reservoirs, any insoluble suspended matter becomes deposited, and the brine then possesses a beautiful transparency, and a flue sea-green colour. In most of the Continental salt-works, the bessoirs already mentioned are employed. These require to be very carefully constructed, and are hest arranged with external tie-rods armed with screws, so as to be able to tighten the joints from time to time.
In England, also, are wooden reservoirs for the brine in and about many of the salt-works, but they are usually fastened with tree-nails, and strengthened by stout timber ribs placed externally and very near each other, and the seams are kept tight by caulking; but they give endless trouble to keep them watertight. There is, however, probably far less loss under any circumstances from the wooden reservoirs than from those of puddled clay paved with stone generally used in Cheshire. Tho action of strong brine upon any wooden vessel in which it is stored, though highly preserva tive to the wood, is peculiarly astringent and contractile, and unless care be taken to provide against the emergency, it will be found that new wooden vessels, no matter how well made or how well the tirnber may have previously been seasoned, will not long stand the action of saturated brine, without requiring their joints to be tightened or caulked.
In cases where the brine is contaminated with any large quantities of magnesium salts, it is nsual t,o decompose these in the bessoirs by an addition of milk of lime, the operation being called °haulage. The lime displaces the magnesium as a flocculent precipitate of hydrated magnesia, and decomposes at the same time any iron or aluminium salts, of which traces are usually present ; it is added in just sufficient proportion for this purpose. As the precipitate accumulates in the bessoirs, they are cleaned out. The °haulage has been abandoned, except where necessity obliges it, for not
only is the brine thereby made alkaline, and the crystallization rendered more difficult, but the liquor takes a long time to clarify, and during evaporation a crust is liable to form on the surface, which seriously interferes with the work. A far better plan is to wash away the magnesian salt, and other impurities that may be present, by a plentiful aspersion of the salt, after it has beau made, with some saturated brine. In England, the brine as pumped and settled is sufficiently pure to be run at once in a continuous stream to the evaporating-pans, producing salt of superior quality, the mother liquor hardly ever requiring t,o be run away till the pan is laid off for repairs (see Table opposite).
The nomenclature of the principal qualities of salt occurring in commerce may be stated as follows :— Group I. Boiled.—Fine salt, sometimes called " lump " or " stoved lump "; superfine, also stoved ; butter salt, not stoved ; cheese salt, not stoved.
Group II. Not Boiled.—Common, fishery, extra fishery, double extra fishery, bay-salt.
In the French nomenclature, are fin fin, sometimes called sel d /a minute, corresponding at the same time to our fine and butter salts ; then 6-, 12-, 24-, 48-, 72-, and 96-hour salts, and another quality called &allies, a very coarse large grain, corresponding t,o bay-salt, chiefly made for sprinkling on the top of the salt fish in barrels.
The classification of the various qualities of salt varies with the size and appearance of its crystals, and these in their turn are almost entirely dependent on the rate at which the evaporation is carried on, and the greater or less degree in which the brine was agitated during that time. The boiled salts, produced during arta tl ebullition, and the liquid being frequently agitated, o.re fine in grain ; the unboiled, produced by slower evapomtion, are of coarser grain, according to the temperature, the tinie expended in their production, and the stillness of the brine during that proce,4. The evaporating-pane are built of common boiler-plate, I-I in. thick, the plates being about 4 ft. long by 2 ft. wide, and well riveted together. The platen are usually of rather amaller dimensions in the part immediEttely over the fire than eleewhere on the bottom or floor of the pan, es by thia mearts some of the tendency to warp and buckle is suppoeed to be avoided. In England, the ueual dimensions for fine aitd extra-fine oalt-pane are 30 ft. long by 22-25 ft. wide, and 1 ft. 9 in. deep. Thin gives an evaporating surface of 720-750 eq. ft. Butter-ealt pane are perhape a trifle longer, eay 35 ft. by 22-25 ft., and the same depth, with an evaporating eurface of 770-875 sq. ft. Common and fishery pans mnge from 50 to 70 by 22-25 ft., and have the same depth, presenting au evaporating surface of 1100-1750 sq. ft.; eome fiehery salt-pans belonging to the British Salt Co. at Anderton aro 90 ft. by 22, while at Stoke and Wineford, are fishery ealt-pans ranging up to 130 ft. in length. Beyond 70 ft. in length, however, there really would not eeem to be eufficient gain, at least with the quality of fuel need in Cheehire, to compensate the increased cost of con struction and repairs. In France, the common and fiohery salt-pans are about the same sizea as ours, only perhaps a trifle wider ; and at Dombasle, near Nancy, where the intelligent manager Botta, has carried the manufacture to as great perfection as is attained in perhaps any worke, the pans (poeles) are 72 ft. by 29i ft. by 43i in., with an evaporating surface of 2124 :1. ft.